Normal function of the immune system depends upon interactions between T and B cells, which take place in the secondary lymphoid organs. In order for the cognate interactions to occur between rare antigen-specific T and B cells, large numbers of cells must circulate through the lymphoid organs. While adhesion pathways for migration to lymph nodes and Peyer's patches have been identified, none has been found to play a major role in migration to the spleen. Using adoptive transfer techniques, we have shown that the time course of migration into the lymphoid compartment of the spleen differs for T and B cells. In addition, we find that entry of activated T lymphocytes into the white pulp of the spleen is retarded. The goal of this application is to provide further insight into the pathways with which T and B lymphocytes migrate into and through the lymphoid compartment of the spleen. To test the hypothesis that T and B lymphocytes are specifically targeted to the lymphoid compartment of the spleen, we propose three specific aims: I. To identify and characterize the locations where T and B lymphocyte migration into the white pulp occurs; II. To identify and characterize key molecules regulating the migration of lymphocytes into the white pulp of the spleen; III. To determine the effect of activation status on lymphocyte homing to the white pulp of the spleen. Experiments will involve adoptive transfer of purified lymphocytes from and into mice deficient in various adhesion molecules. The migration of adoptively transferred lymphocytes will be visualized by intravital microscopy using a real-time, scanning confocal microscope. Antibodies recognizing spleen-specific structures will be prepared and tested for their ability to modify lymphocyte migration behavior in this tissue. Comparison of lymphocyte migration in activated and non-activated cells is planned. We will use RNase protection assays to elucidate potential chemokines and receptors that are important in the homing. Additionally, using a transgene approach, various adhesion molecules, as well as chemokine receptors will be over-expressed in cultured lymphocytes. These cells will then be used in adoptive transfer experiments to illuminate their role in trafficking to the spleen. Identification of mechanisms controlling entry of lymphocytes into the lymphoid compartment of the spleen will provide new targets for immunomodulatory drug therapy.